The present invention relates to a printing press and, more particularly, to a printing press having a on-machine plate making function of exposing an image on a printing plate mounted on a plate cylinder and an ink film thickness control function.
Recently, to improve the efficiency of plate making and register accuracy, a printing press has been designed to directly perform plate making by using a plate making apparatus incorporated in the printing press itself. That is, a graphic pattern (image) is exposed on a printing plate (raw plate) mounted on a plate cylinder by irradiating it with a laser beam from the head of a plate making apparatus incorporated in a printing unit instead of using a plate making apparatus provided independently of the printing press. This operation is called on-machine plate making.
More specifically, the printing press is accelerated to a designated rotational speed. When the rotational speed is stabilized, laser radiation (exposure) from the head to a raw plate is started. Thereafter, the head is moved in the axial direction of the plate cylinder during exposure to expose an image on the entire plate. The exposure time is determined by the size of the plate and the designated rotational speed during exposure. As a technique of exposing an image on a raw plate by laser radiation, the technique disclosed in U.S. Pat. No. 5,379,698 (reference 1) is incorporated in this specification.
FIG. 8 shows how plate making apparatuses are incorporated in a four-color web offset printing press. Referring to FIG. 8, reference numerals 1-1 to 1-4 denote printing units for the respective ink colors. Plate making apparatuses 2-1 to 2-4 are respectively incorporated in the printing units 1-1 to 1-4. The plate making apparatuses 2-1 to 2-4 are normally located at the positions indicated by the chain double-dashed lines in FIG. 8 and brought near to plate cylinders 3 in the printing units 1-1 to 1-4 when exposure is performed. Reference numeral 4 denotes a blanket cylinder which is in contact opposite to the plate cylinder and on which a blanket is mounted. Impression cylinders (not shown) are respectively placed below the blanket cylinders 4.
FIG. 9 shows the main part of a plate making apparatus 2. The plate making apparatus 2 includes an exposure apparatus 2b having a head 2a. The exposure apparatus 2b is fixed on a table 2c. The table 2c moves in the axial direction (the direction indicated by arrows A and B in FIG. 9) of the plate cylinder 3 while being guided by rails 2f1 and 2f2 on a stage 2f. A raw plate 5 before plate making is mounted on the plate cylinder 3.
[Ink Film Thickness Control]
To adjust the amount of ink to be supplied and decrease the number of times of test printing until a desired tone is obtained, an ink film thickness control method like that disclosed in U.S. Pat. Nos. 5,884,562 (reference 2) and 5,921,184 (reference 3) has been proposed. These references disclose ink film thickness control methods called xe2x80x9cpre-inking 1xe2x80x9d and xe2x80x9cpre-inking 2xe2x80x9d. According to the ink film thickness control methods disclosed in these references, when a printing plate is set in the plate cylinder for the first time, an ink film thickness distribution is formed in the inking device by xe2x80x9cpre-inking 1xe2x80x9d. When an old printing plate is to be changed to a new printing plate, an ink film thickness distribution is formed in the inking device by xe2x80x9cpre-inking 2xe2x80x9d. The technique disclosed in references 1 and 2 is incorporated in this specification.
FIG. 10 shows the main part of the inking device (inker) in a printing unit 1. Reference numeral 6 denotes an ink fountain; 7, an ink stored in the ink fountain 6; 8, an ink fountain roller; 9, a plurality of ink fountains aligned in the axial direction of the ink fountain roller 8; 10, an ink ductor roller; 11, an ink roller group; and 12, a printing plate on which an image has already been exposed.
In the inking device having this arrangement, the ink 7 is supplied from the ink fountain 6 onto the surface of the ink fountain roller 8 through a portion between the ink fountain key 9 and the ink fountain roller 8. The ink supplied to the ink fountain roller 8 is supplied to the printing plate 12 via the ink roller group 11 upon ink feed operation of the ink ductor roller 10. The ink supplied to the printing plate 12 is printed on printing paper.
When the old printing plate is changed to the new printing plate 12, the opening amount of the ink fountain key 9, the rotation amount of the ink fountain roller 8, and the like are preset to values corresponding to the image on the printing plate 12. More specifically, by setting the opening amount of the ink fountain key 9, the rotation amount of the ink fountain roller 8, and the like to the values corresponding to the image on the printing plate 12, the ink 7 in the ink fountain 6 is supplied to the printing plate 12 via the ink roller group 11. In this case, test printing is performed before final printing to obtain a satisfactory tone while adjusting the amount of ink to be supplied. With this operation, a desired ink film thickness distribution (ink film thickness gradient) is formed on the ink roller group 11.
When the old printing plate is changed to the new printing plate 12, the ink film thickness distribution is left on the ink roller group 11. For the new printing plate 12, this ink film thickness distribution for the old printing plate must be gradually changed to an ink film thickness distribution suited to the new printing plate 12. For this reason, to obtain a satisfactory tone, adjustment of the amount of ink to be supplied and test printing are required to excessive degrees, resulting in problems, e.g., an increase in printing preparation time, an increase in work load, a waste of printing materials, a decrease in production efficiency, and an increase in cost.
According to references 2 and 3 described above, when the old printing plate is to be changed to the printing plate 12, ink removing operation is performed first. More specifically, ink removing is selected on a display (not shown) after a printing unit is selected. In ink removing operation, the ink feed operation of the ink ductor roller 10 is set in the OFF state, and the printing press is driven while the old printing plate is mounted to print out a predetermined number of sheets. With this operation, as shown in FIG. 11A, a minimum ink film thickness distribution Ma required during printing is left on the ink roller group 11, which decreases in thickness from upstream to downstream. That is, the basic ink film thickness distribution Ma corresponding to a portion of the printing plate 12 which has no image is left.
Pre-inking 2 is then selected on the display to perform operation of pre-inking 2. In pre-inking 2, after the opening amount of the ink fountain key 9, the rotation amount of the ink fountain roller 8, and the like are preset to values corresponding to the image on the printing plate 12, the printing press is driven, and the ink feed operation of the ink ductor roller 10 is performed a predetermined number of times. With this operation, as shown in FIG. 11B, an ink film thickness distribution (to be referred to as image ink film thickness distribution hereinafter) Mb corresponding to the image on the printing plate 12 is superimposed on the basic ink film thickness distribution Ma left on the ink roller group 11.
After the image ink film thickness distribution Mb is superimposed on the basic ink film thickness distribution Ma, test printing corresponding to a predetermined number of sheets is performed while the printing plate is changed to the new printing plate 12, thereby performing density checks on printing products produced by test printing. In the density checks, if a satisfactory tone is obtained, ink film thickness control by xe2x80x9cpre-inking 2xe2x80x9d is terminated, and final printing is started.
If the ink roller group 11 holds no ink, e.g., the printing plate 12 is mounted on the surface of the plate cylinder 3 for the first time, a printing unit is selected on the display first, and then pre-inking 1 is selected. In pre-inking 1, the total opening amount of the ink fountain keys 9 is initialized to a reference opening amount (e.g., 50%), and the rotation amount of the ink fountain roller 8 is initialized to a reference rotation amount (e.g., 50%). In this state, the printing press is driven, and the ink feed operation of the ink ductor roller 10 is performed a predetermined number of times to form the basic ink film thickness distribution Ma on the ink roller group 11. After the basic ink film thickness distribution Ma is formed, the opening amount of the ink fountain key 9 and the rotation amount of the ink fountain roller 8 are preset to values corresponding to the image on the new printing plate 12. The ink feed operation of the ink ductor roller 10 is then performed a predetermined number of times to superimpose the image ink film thickness distribution Mb corresponding to the printing plate 12 on the basic ink film thickness distribution Ma formed on the ink roller group 11.
In this state, test printing corresponding to a predetermined number of sheets is performed, and density checks are made on printing products produced by test printing. In these density checks, if a satisfactory tone is obtained, ink film thickness control by xe2x80x9cpre-inking 1xe2x80x9d is terminated, and final testing is started.
[On-machine Plate Making+Ink Film Thickness Control]
A series of operations ranging from plate making to final printing, including the above on-machine plate making and ink film thickness control, are serially performed. FIG. 12A shows conventional steps in on-machine plate making including a preparatory process (ink removing, cleaning of the impression cylinder and blanket, paper size/paper thickness preset operation, plate change, and the like) and forming an ink film thickness distribution by pre-inking 2. FIG. 12B shows conventional steps in on-machine plate making without any preparatory process and forming an ink film thickness distribution by pre-inking 2.
Referring to FIG. 12A, first of all, ink removing is performed while an old printing plate is mounted on the plate cylinder 3 (step S21) to leave the basic ink film thickness distribution Ma on the ink roller group 11. After ink removing, the impression cylinder and blanket are cleaned (step S22). Concurrently with this cleaning operation, a paper size/paper thickness is preset (step S23). An automatic plate change unit (not shown) is driven to change the old printing plate mounted on the plate cylinder 3 with a raw plate (step S24).
The plate making apparatus 2 is then driven to perform exposure, thereby exposing an image on the raw plate 5 (step S25). Pre-inking 2 is performed (step S26) to superimpose the image ink film thickness distribution Mb corresponding to the image exposed on the raw plate 5 on the basic ink film thickness distribution Ma left on the ink roller group 11. After the image ink film thickness distribution Mb is superimposed, test printing is performed (step S27). If a satisfactory tone is obtained, the flow advances to final printing (step S28).
Referring to FIG. 12B, since the above preparatory processes, i.e., the operations in steps S21 to S24 in FIG. 12A, have been completed, the subsequent process is performed from the exposure in step S25. After an image is exposed on the raw plate 5 by the exposure in step S25, pre-inking 2 is performed (step S26). Test printing is then performed (step S27), and the flow advances to final printing (step S28). The exposure time in step S25 is determined by the size of a plate and a designated rotational speed in exposure. According to a conventional, standard plate making method, the exposure time is about 3 min and 40 sec. According to a conventional, standard ink film thickness control method, it takes about 1 min and 30 sec to form an ink film thickness distribution by pre-inking 2 in step S26.
FIGS. 13A and 13B show conventional steps in serially performing on-machine plate making and ink film thickness distribution formation by pre-inking 1. FIG. 13A shows conventional steps in on-machine plate making and ink film thickness distribution formation by pre-inking 2 with a preparatory process. FIG. 13B shows conventional steps in on-machine plate making and ink film thickness distribution formation by pre-inking 2 without any preparatory process.
Referring to FIG. 13A, the flow starts with a preparatory process including cleaning of the impression cylinder, blanket, and inker, paper size/paper thickness presetting, and plate change. That is, the impression cylinder, blanket, and inker are cleaned (step S11). Concurrently with this cleaning operation, paper size/paper thickness presetting is performed (step S12). The old printing plate mounted on the plate cylinder 3 is changed to the raw plate 5 by using the automatic plate change unit (step S13). The plate making apparatus 2 is then driven to perform exposure so as to expose an image on the raw plate 5 (step S14). Pre-inking 1 is performed (step S15) to form the ink film thickness distributions Ma and Mb. After the formation of the ink film thickness distributions Ma and Mb, test printing is performed (step S16). If a satisfactory tone is obtained, the flow advances to final printing (step S17).
Referring to FIG. 13B, since the operations in steps S11 to S13 in FIG. 13A have already been completed, the flow starts with exposure in step S14. After an image is formed on the raw plate 5 by exposure in step S14, pre-inking 1 is performed (step S15). After test printing is performed (step S16), the flow advances to final printing (step S17). The exposure time in step S14 is determined by the size of a plate and the designated rotational speed in exposure. According to a conventional, standard plate making method, the exposure time is about 3 min and 40 sec. It takes about 2 min and 30 sec to form an ink film thickness distribution by pre-inking 1 in step S15.
In the processes shown in FIGS. 12A and 12B, an ink film thickness distribution is formed (step S26) between exposure (step S25) and test printing (step S27). In the processes shown FIGS. 13A and 13B, an ink film thickness distribution is formed (step S15) between exposure (step S14) and test printing (step S16).
In this case, the time taken for ink film thickness distribution formation is added to the exposure time, resulting in the prolongation of the time from plate making to final printing. In recent printing, there is a tendency to produce many types of printing plates in small lots. If, therefore, exposure in on-machine plate making is performed, it takes much time to start final printing, resulting in a significant loss in terms of operation efficiency.
It is an object of the present invention to provide a printing press which shortens the time taken until final printing in continuous execution of on-machine plate making and ink film thickness distribution formation.
In order to achieve the above object, according to the present invention, there is provided a printing press comprising an ink roller group for supplying ink stored in an ink fountain to a printing plate mounted on a plate cylinder, image exposing means for exposing an image on a printing plate before plate making which is mounted on the plate cylinder, ink film thickness distribution formation means for forming an ink film thickness distribution corresponding to the image to be exposed on the printing plate on the ink roller group, and control means for performing image exposing and ink film thickness distribution formation concurrently at least partly by controlling the image exposing means and the ink film thickness distribution formation means.